Cut roast in half. Place in a 5-qt. slow cooker. In a small bowl, combine the ketchup, onion, brown sugar, vinegar, mustard, Worcestershire sauce, garlic, salt, celery seed, paprika and pepper; pour over roast. Cover and cook on low for 8-10 hours or until meat is tender.
Remove meat. Skim fat from cooking juices; transfer to a large saucepan. Bring to a boil. Combine cornstarch and water until smooth; gradually stir into juices. Return to a boil; cook and stir for 2 minutes or until thickened.
When meat is cool enough to handle, shred with two forks. Return to slow cooker and stir in sauce mixture; heat through. Serve on rolls with pickle slices if desired.